Video prediction encoding device, video prediction encoding method, video prediction decoding device and video prediction decoding method

ABSTRACT

A decoding device includes a decoder that decodes information of a direction of intra-picture prediction of a target block and compression data of a residual signal, a prediction signal generator that generates an intra-picture prediction signal using the information of the direction and an previously reconstructed reference sample of an adjacent block, a residual signal restorator that restores a reconstructed residual signal of the target block, and a block storage that restores and stores a pixel signal of the target block. The prediction signal generator derives reference samples from a previously reconstructed block neighboring the target block stored, selects two or more key reference samples, performs an interpolation process between the key reference samples for generating interpolated reference samples, and generates the intra-picture prediction signal by extrapolating the interpolated reference samples based on the direction of the intra-picture prediction.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/665,545 filed on Mar. 23, 2015, which is a continuation ofPCT/JP2013/066616 filed on Jun. 17, 2013, which claims priority toJapanese Application No. 2012-209626 filed on Sep. 24, 2012. The entirecontents of these applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a moving picture prediction encodingdevice and method, and a moving picture prediction decoding device andmethod, and more particularly to filter processing of neighbouringsamples for use in intra-picture prediction.

BACKGROUND ART

Compression encoding technologies are used to efficiently transmit andaccumulate moving picture data. MPEG-1 to 4 and H.261 to H.264 arewidely used video coding technologies.

In such video coding technologies, encoding processing and decodingprocessing are carried out after an image to be encoded is divided intoa plurality of blocks. In intra-picture prediction encoding, aprediction signal is generated using a previously reconstructedneighbouring image signal (obtained by restoring compressed image data)located within the current picture where a target block is included, andthereafter a differential signal is obtained by subtracting theprediction signal from the signal of the target block and encoded. Ininter-picture prediction encoding, referring to a previouslyreconstructed image signal within a picture different from the picturewithin which the target block is included, motion compensation iscarried out, and a prediction signal is generated. The prediction signalis subtracted from the signal of the target block to generate adifferential signal, and the differential signal is encoded.

Ordinarily, in inter-picture prediction (inter prediction) encoding, aprediction signal is generated by searching previously reconstructedpictures for a signal resembling the pixel signal of a block to beencoded previously. A motion vector that represents the spatialdisplacement amount between the target block and the region formed bythe signal searched for, and the residual signal between the pixelsignal of the target block and the prediction signal are encoded. Thetechnique of searching respective bocks for the motion vector in thisway is called block matching.

FIG. 10 is a schematic diagram for explaining the block matchingprocess. Here, the procedure for generating a prediction signal isdescribed with an example in which a picture 701 includes a target block702 to be encoded. A reference picture 703 has previously beenreconstructed. A region 704 is located at the spatially same position asthe target block 702 is located. In the block matching process, a searchregion 705 neighbouring the region 704 is defined, and from the pixelsignals in the search region, a region 706 is to be detected that hasthe lowest sum of the absolute differences from the pixel signals of thetarget block 702. The signal of the region 706 becomes a predictionsignal, and the displacement amount from the region 704 to the region706 is detected as a motion vector 707. Furthermore, a method iscommonly used in which a plurality of reference pictures 703 isidentified for each target block, a reference picture is selected onwhich the block matching is performed, and reference picture selectioninformation is generated. In H.264, in order to cope with local featurechanges in images, a plurality of prediction types are provided whichare used with different block sizes each for encoding a motion vector.The prediction types of H.264 are described, for example, in PatentLiterature 2.

H264 also performs intra-picture prediction (intra prediction) encodingin which a prediction signal is generated by extrapolating, inpredetermined directions, the values of the previously reconstructedpixels adjacent to a block to be encoded. FIG. 11 is a schematic diagramfor explaining the intra-picture prediction used in ITU H.264. In FIG.11(A), a target block 802 is a block to be encoded, and a pixel group(reference sample group) 801 is from an adjacent region which includesimage signal previously reconstructed in previous processing, and thegroup includes pixels A to M adjacent to the boundary of the targetblock 802 previously reconstructed.

In this case, a prediction signal is generated by extending the pixelgroup (reference sample group) 801 of adjacent pixels immediately abovethe target block 802 in the downward direction. In FIG. 11(B), aprediction signal is generated by extending the previously reconstructedpixels (I to L) located on the left of a target block 804 in therightward direction. A detailed explanation for generating a predictionsignal is given, for example, in Patent Literature 1. The differencefrom the pixel signal of the target block is calculated for each of thenine prediction signals generated as shown in FIGS. 11(A)-11(B). Theprediction signal having the smallest difference value is selected asthe optimum prediction signal. As described above, prediction signals(intra prediction samples) can be generated by extrapolating the pixels.The description above is provided in Patent Literature 1 below.

The intra-picture prediction shown in Non Patent Literature 1 provides25 types of prediction signal generation methods all performed indifferent directions of extending reference samples, in addition to the9 types described above (a total of 34 types).

In Non Patent Literature 1, in order to suppress distortions inreference samples, the reference samples are subjected to a low passfilter before a prediction signal is generated. Specifically, a 121filter having weight coefficients of 1:2:1 is applied to the referencesamples before the extrapolation prediction. This processing is calledintra smoothing.

With reference to FIG. 7 and FIG. 8, the intra-picture prediction in NonPatent Literature 1 is described. FIG. 7 shows an example of blockdivision. Five blocks 220, 230, 240, 250, and 260 adjacent to a targetblock 210, which has a block size of N×N samples, have previously beenreconstructed. For intra prediction of the target block 210, referencesamples denoted as ref[x] (x=0 to 4N) are used. FIG. 8 shows the processflow of the intra prediction. First, in step 310, reference samplesref[x] (x=0 to 4N) are derived from a memory into which a predictionsignal generator for carrying out the intra-picture prediction processstores reconstructed pixels. In the step, some of the adjacent blocksmay not have been reconstructed because of the encoding order, and allthe 4N+1 samples ref[x] may not be derived. If it is the case, themissing samples are substituted with samples generated by a paddingprocess (the values of the neighbouring samples are copied), whereby4N+1 reference samples are prepared. The details of the padding processare described in Non Patent Literature 1. Next, in step 320, theprediction signal generator performs the smoothing process on thereference samples using the 121 filter. Finally, in step 330, theprediction signal generator predicts a signal in the target block byextrapolations (in the directions of intra-picture prediction) andgenerates a prediction signal (i.e., intra prediction samples).

CITATION LIST Patent Literature

-   Patent Literature 1: U.S. Pat. No. 6,765,964-   Patent Literature 2: U.S. Pat. No. 7,003,035

Non Patent Literature

-   Non Patent Literature 1: B. Bross et al., “High efficiency video    coding (HEVC) text specification draft 8”, Joint Collaborative Team    on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC    JTC1/SC29/WG11, JCTVC-J1003, 10th Meeting: Stockholm, Sweden, 11-20    Jul. 2012.

SUMMARY OF THE INVENTION Technical Problem

FIG. 9 shows an example of a signal representing a flat region in whichpixel values are similar. When the original pixel values (originalsample values) 410 are encoded by coarse quantization, reconstructedvalues (reconstructed sample values) 420 in the block take a constantvalue, and a step-like distortion appears at a block boundary 430. Thisdistortion is known as block noise and is usually removed by applying ablock noise removing filter to the reconstructed image. However, thereference sample used in intra-picture prediction is a signal preparedpreviously to the application of the filter process for removing blocknoise, so that the block noise remaining in the reference sample at theblock boundary propagates to the prediction signal (intra predictionsamples) of the target block through intra-picture prediction. The blocknoise that has propagated to the prediction signal cannot be removed bya block noise removal process for a reconstructed signal and thereforepropagates directly to the reference sample group for the next targetblock.

In Non Patent Literature 1, 34 different types of extrapolationdirections are prepared in the extrapolation method of intra-pictureprediction (in the directions of intra-picture prediction), so thatblock noise propagates while changing directions. As a result, aplurality of contouring artifacts are produced in the reconstructedsignal of a flat region in an image. In particular, when noisepropagates to a block of a large size, contouring artifacts run acrossthe large block, giving an unpleasant visual effect.

The 121 filter described in Background Art can effectively remove noisewithin reference samples but cannot remove step-like noise asillustrated in FIG. 9 because of a short tap number.

The object of the present invention is to suppress artificial noise suchas the contouring artifacts described above.

Solution to Problem

A moving picture prediction encoding device according to an aspect ofthe present invention includes a block divider that divides an inputimage into a plurality of blocks, a prediction signal generator thatgenerates, using previously reconstructed reference samples locatedadjacent to a target block to be encoded among the divided blocks fromthe block divider, an intra-picture prediction signal of a block havinga higher correlation with the target block previously reconstructed. Themoving picture prediction encoding device further includes a residualsignal generator that generates a residual signal between the predictionsignal of the target block and the pixel signal of the target block, aresidual signal compressor that compresses the residual signal generatedby the residual signal generator, a residual signal restorator thatgenerates a reconstructed residual signal by restoring the compressedresidual signal, an encoder that encodes the compressed residual signal,and a block storage that restores the pixel signal of the target blockby adding the prediction signal to the reconstructed residual signal,and stores the restored pixel signal of the target block to be used asreference samples. The prediction signal generator derives referencesamples from previously reconstructed blocks, stored in the blockstorage, which surround the target block, selects two or more keyreference samples from the reference samples, performs an interpolationprocess between the key reference samples for generating interpolatedreference samples, determines a direction of intra-picture prediction,and generates the intra-picture prediction signal by extrapolating theinterpolated reference samples based on the determined direction of theintra-picture prediction. The encoder encodes information of thedirection of intra-picture direction together with the compression dataof the residual signal.

In the moving picture prediction encoding device described above, theprediction signal generator may selectively carry out the interpolationprocess of the reference samples or a smoothing process of the referencesamples, based on a comparison between the key reference samples and apredetermined threshold.

In the moving picture prediction encoding device described above, thereference samples may be such reference samples as located at the end ofthe reference sample group, and the interpolation process may be abilinear interpolation process performed on the reference samplesbetween the key reference samples.

A moving picture prediction decoding device according to an aspect ofthe present invention includes a decoder that decodes, from encodedcompression data for a plurality of divided blocks, information of adirection of intra-picture prediction to be used in intra-pictureprediction of a target block to be decoded and a compressed residualsignal, a prediction signal generator that generates an intra-pictureprediction signal using the information of the direction of theintra-picture prediction and previously reconstructed reference sampleslocated adjacent to the target block, a residual signal restorator thatrestores a reconstructed residual signal of the target block from thecompressed residual signal, and a block storage that restores a pixelsignal of the target block by adding the prediction signal to thereconstructed residual signal, and stores the restored pixel signal ofthe target block to be used as reference samples. The prediction signalgenerator derives reference samples from previously reconstructedblocks, stored in the block storage, which neighbour the target block,selects two or more key reference samples from the reference samples,performs an interpolation process between the key reference samples forgenerating interpolated reference samples, and generates theintra-picture prediction signal by extrapolating the interpolatedreference samples based on the direction of the intra-pictureprediction.

In the moving picture prediction decoding device described above, theprediction signal generator may selectively carry out an interpolationprocess of the reference samples or a smoothing process of the referencesamples, based on a comparison between the key reference samples and apredetermined threshold.

In the moving picture prediction decoding device described above, thereference samples may be such reference samples as located at the end ofa reference sample group, and the interpolation process may be abilinear interpolation process performed on the reference samplesbetween the key reference samples.

The present invention may be taken as relating to a moving pictureprediction encoding method, to a moving picture prediction decodingmethod, to a moving picture prediction encoding program, and to a movingpicture prediction decoding program, and can be described as follows.

A moving picture prediction encoding method according to an aspect ofthe present invention is executed by a moving picture predictionencoding device. The moving picture prediction encoding method includesa block division step of dividing an input image into a plurality ofblocks, a prediction signal generation step of generating, usingpreviously reconstructed reference samples located adjacent to a targetblock to be encoded among the divided blocks from the block divisionstep, an intra-picture prediction signal of a block having a highercorrelation with the target block previously reconstructed, a residualsignal generation step of generating a residual signal between theprediction signal of the target block and the pixel signal of the targetblock, a residual signal compression step of compressing the residualsignal generated in the residual signal generation step, a residualsignal restoration step of generating a reconstructed residual signal byrestoring the compressed residual signal, an encoding step of encodingthe compressed residual signal, and a block storage step of restoringthe pixel signal of the target block by adding the prediction signal tothe reconstructed residual signal, and storing the restored pixel signalof the target block to be used as reference samples. In the predictionsignal generation step, reference samples are derived from previouslyreconstructed blocks, which are stored and neighbour the target block,two or more key reference samples are selected from the referencesamples, an interpolation process is performed between the key referencesamples for generating interpolated reference samples, a direction ofintra-picture prediction is determined, and the intra-picture predictionsignal is generated by extrapolating the interpolated reference samplesbased on the determined direction of the intra-picture prediction. Inthe encoding step, information of the direction of the intra-pictureprediction is encoded together with the compression data of the residualsignal.

A moving picture prediction decoding method according to an aspect ofthe present invention is executed by a moving picture predictiondecoding device. The moving picture prediction decoding method includesa decoding step of decoding, from encoded compression data for aplurality of divided blocks, information of a direction of intra-pictureprediction to be used in intra-picture prediction of a target block tobe decoded and a compressed residual signal, a prediction signalgeneration step of generating an intra-picture prediction signal usingthe information of the direction of the intra-picture prediction andpreviously reconstructed reference samples located adjacent to thetarget block, a residual signal restoration step of restoring areconstructed residual signal of the target block from the compressedresidual signal, and a block storage step of restoring a pixel signal ofthe target block by adding the prediction signal to the reconstructedresidual signal, and storing the restored pixel signal of the targetblock to be used as reference samples. In the prediction signalgeneration step, reference samples are derived from previouslyreconstructed blocks, which are stored and neighbour the target block,two or more key reference samples are selected from the referencesamples, an interpolation process is performed between the key referencesamples for generating interpolated reference samples, and theintra-picture prediction signal is generated by extrapolating theinterpolated reference samples based on the direction of theintra-picture prediction.

A moving picture prediction encoding program according to an aspect ofthe present invention causes a computer to function as a block dividerthat divides an input image into a plurality of blocks, a predictionsignal generator that generates, using previously reconstructedreference samples located adjacent to a target block to be encoded amongthe divided blocks from the block divider, an intra-picture predictionsignal of a block having a higher correlation with the target blockpreviously reconstructed, a residual signal generator that generates aresidual signal between the prediction signal of the target block andthe pixel signal of the target block, a residual signal compressor thatcompresses the residual signal generated by the residual signalgenerator, a residual signal restorator that generates a reconstructedresidual signal by restoring the compressed residual signal, an encoderthat encodes the compression data of the residual signal, and a blockstorage that restores the pixel signal of the target block by adding theprediction signal to the reconstructed residual signal, and stores therestored pixel signal of the target block to be used as the referencesample. The prediction signal generator derives reference samples frompreviously reconstructed blocks, stored in the block storage, whichneighbour the target block, selects two or more key reference samplesfrom the reference samples, performs an interpolation process betweenthe key reference samples for generating interpolated reference samples,determines a direction of intra-picture prediction, and generates theintra-picture prediction signal by extrapolating the interpolatedreference samples based on the determined direction of the intra-pictureprediction. The encoder encodes information of the direction ofintra-picture direction together with the compression data of theresidual signal.

A moving picture prediction decoding program according to an aspect ofthe present invention causes a computer to function as a decoder thatdecodes, from encoded compression data for a plurality of dividedblocks, information of a direction of intra-picture prediction to beused in intra-picture prediction of a target block to be decoded and acompressed residual signal, a prediction signal generator that generatesan intra-picture prediction signal using the information of thedirection of the intra-picture prediction and previously reconstructedreference samples located adjacent to the target block, a residualsignal restorator that restores a reconstructed residual signal of thetarget block from the compressed residual signal, and a block storagethat restores the pixel signal of the target block by adding theprediction signal to the reconstructed residual signal, and stores therestored pixel signal of the target block to be used as referencesamples. The prediction signal generator derives reference samples frompreviously reconstructed blocks, stored in the block storage, whichneighbour the target block, selects two or more key reference samplesfrom the reference samples, performs an interpolation process betweenthe key reference samples for generating interpolated reference samples,and generates the intra-picture prediction signal by extrapolating theinterpolated reference samples based on the direction of theintra-picture prediction.

Effects of the Invention

With the filter process applied on the reference samples by bilinearinterpolation in accordance with the present invention, the signals inthe reference samples are made gradually changed using samples at bothends of the reference samples, thereby suppressing such artificial noiseas contouring artifacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a moving picture prediction encodingdevice according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a moving picture prediction decodingdevice according to the embodiment of the present invention.

FIG. 3 is a flowchart showing an intra-picture prediction methodaccording to the embodiment of the present invention.

FIG. 4 is a flowchart showing another example of the intra-pictureprediction method according to the embodiment of the present invention.

FIG. 5 is a diagram showing a hardware configuration of a computer forexecuting a program stored in a recording medium.

FIG. 6 is an overview of the computer for executing a program stored ina recording medium.

FIG. 7 is a diagram illustrating an example of reference samples used inintra-picture prediction.

FIG. 8 is a flowchart showing an intra-picture prediction method in aconventional technique.

FIG. 9 is a diagram illustrating the relation between an original signaland a reconstructed signal in a flat region.

FIG. 10 is a schematic diagram for explaining a motion estimationprocess in inter-picture prediction.

FIG. 11 is a schematic diagram for explaining intra-picture predictionby extrapolation of reference samples.

FIG. 12 is a diagram illustrating another example of reference samplesused in intra-picture prediction.

FIG. 13 is a flowchart illustrating a process in a prediction signalgenerator 103 in FIG. 1.

FIG. 14 is a flowchart illustrating a process in a prediction signalgenerator 208 in FIG. 2.

FIG. 15 is a flowchart showing a second another example of theintra-picture prediction method according to the embodiment of thepresent invention.

FIG. 16 is a block diagram showing a configuration of a moving pictureprediction encoding program.

FIG. 17 is a block diagram showing a configuration of a moving pictureprediction decoding program.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to FIG. 1 to FIG. 7 and FIG. 13 to FIG. 17.

FIG. 1 is a block diagram showing a moving picture prediction encodingdevice 100 according to an embodiment of the present invention. As shownin FIG. 1, the moving picture prediction encoding device 100 includes aninput terminal 101, a block divider 102, a prediction signal generator103, a frame memory 104, a subtractor 105, a transformer 106, aquantizer 107, an inverse quantizer 108, an inverse transformer 109, anadder 110, an entropy encoder 111, an output terminal 112, a blockmemory 113, and a loop filter 114. The subtractor 105, the transformer106, and the quantizer 107 function as “an encoder” recited in theclaims. The inverse quantizer 108, the inverse transformer 109, and theadder 110 function as “a decoder” recited in the claims. The framememory 104 functions as “an image storage”, and the block memory 113functions as “a block storage”.

The operation of the moving picture prediction encoding device 100configured as described above is described below. The signal of a movingpicture composed of a plurality of images is input to the input terminal101. The block divider 102 divides an image to be encoded into aplurality of regions. In the embodiment according to the presentinvention, as shown in the example in FIG. 7, the block size is notlimited. A variety of block sizes and shapes may be coincidently definedin a picture. The block encoding order is described, for example, in NonPatent Literature 1. Next, a prediction signal is generated for a region(hereinafter called “target block”). In the embodiment according to thepresent invention, two types of prediction methods are used, namely,inter-picture prediction and intra-picture prediction. The predictionsignal generation process in the prediction signal generator 103 isdescribed later using FIG. 13.

The subtractor 105 subtracts a prediction signal (through a line L103)from the signal of a target block (through a line L102) to generate aresidual signal. The transformer 106 discrete-cosine transforms theresidual signal. The quantizer 107 quantizes each transform coefficient.The entropy encoder 111 encodes the quantized transform coefficients andoutputs, to the output terminal 112, the encoded transform coefficientsalong with prediction information required to generate a predictionsignal.

In order to perform the intra-picture prediction or the inter-pictureprediction on the subsequent target block, the compressed signal of thetarget block is inversely processed and decoded. More specifically, thequantized transform coefficients are inverse quantized by the inversequantizer 108 and thereafter inversely discrete-cosine transformed bythe inverse transformer 109, whereby the residual signal isreconstructed. The adder 110 adds the reconstructed residual signal tothe prediction signal sent through the line L103 to reproduce the signalof the target block. The signal of the reconstructed block is stored inthe block memory 113 for intra-picture prediction. A reconstructed imageformed of the reconstructed signal is stored in the frame memory 104after a block noise suffered in the reconstructed image is removed bythe loop filter 114.

With reference to FIG. 13, the prediction signal process flow performedin the prediction signal generator 103 is explained. First, in stepS302, prediction information required for inter-picture prediction isgenerated. Specifically, a reconstructed image that is previouslyencoded and thereafter reconstructed is used as a reference image. Thisreference image is searched for a motion vector and a reference picturethat gives a prediction signal with the smallest difference from thetarget block. In this case, the target block is input through the lineL102, and the reference image is input through a line L104. A pluralityof images previously encoded and reconstructed are used as referenceimages. The details thereof are the same as in H.264 which is theconventional technique or method shown in Non Patent Literature 1.

In step S303, prediction information required for intra-pictureprediction is generated. As shown in FIG. 7, the previouslyreconstructed pixel values spatially adjacent to the target block areused to generate prediction signals in a plurality of intra-predictiondirections. Then, the prediction direction (intra prediction mode) thatgives a prediction signal with the smallest difference from the targetblock is selected. Here, the prediction signal generator 103 generatesan intra-picture prediction signal by acquiring the previouslyreconstructed pixel signals within the same picture as reference samplesfrom the block memory 113 through a line L113 and extrapolating thesesignals.

Next, in step S304, a prediction method to be applied to the targetblock is selected from inter-picture prediction and intra-pictureprediction. For example, one of the prediction methods that gives aprediction value with a small difference from the target block isselected. Alternatively, the two prediction methods may be actuallyperformed until the end of the encoding processing, and the one may beselected which has a smaller evaluation value calculated from therelation between the produced encoding amount and the sum of absolutevalues of the encoded difference images. The selection information ofthe selected prediction method is sent as information required togenerate a prediction signal to the entropy encoder 111 through a lineL112 for encoding and is then output from the output terminal 112 (stepS305).

If the prediction method selected in step S306 is inter-pictureprediction, a prediction signal is generated in step S307 based onmotion information (the motion vector and the reference pictureinformation). The generated inter-picture prediction signal is output tothe subtractor 105 through the line L103. In step S308, the motioninformation is sent as the information required to generate a predictionsignal to the entropy encoder 111 through the line L112 for encoding andis then output from the output terminal 112.

If the prediction method selected in step S306 is intra-pictureprediction, a prediction signal is generated in step S309 based on theintra prediction mode. The generated intra-picture prediction signal isoutput to the subtractor 105 through the line L103. In step S310, theintra prediction mode is sent as the information required to generate aprediction signal to the entropy encoder 111 through the line L112 forencoding and is then output from the output terminal 112.

The encoding method used in the entropy encoder 111 may be arithmeticencoding or may be variable length encoding.

FIG. 2 is a block diagram of a moving picture prediction decoding device200 according to an embodiment of the present invention. As shown inFIG. 2, the moving picture prediction decoding device 200 includes aninput terminal 201, a data analyzer 202, an inverse quantizer 203, aninverse transformer 204, an adder 205, a prediction signal generator208, a frame memory 207, an output terminal 206, a loop filter 209, anda block memory 215. The inverse quantizer 203 and the inversetransformer 204 function as “a decoder” recited in the claims. Any otherfunctional units may be used as the decoder. The inverse transformer 204may be omitted. The frame memory 207 functions as “an image storage”,and the block memory 215 functions as “a block storage”.

The operation of the moving picture prediction decoding device 200configured as described above is described below. The compressed datathat is compression encoded by the method described above is input fromthe input terminal 201. The compressed data includes a residual signalobtained by predicting and encoding a target block of a plurality ofblocks from a divided image, as well as the information required togenerate a prediction signal. As shown in the example in FIG. 7, theblock size is not limited. A variety of block sizes and shapes may becoincidently defined in a picture. The block decoding order isdescribed, for example, in Non Patent Literature 1. The informationrequired to generate a prediction signal includes the prediction methodselection information and the motion information (for inter-pictureprediction) or the intra prediction mode (for intra-picture prediction).

The data analyzer 202 decodes the residual signal of the target block,the information required to generate a prediction signal, and thequantization parameter from the compressed data. The inverse quantizer203 inversely quantizes the decoded residual signal of the target blockbased on the quantization parameter (through a line L202). The inversetransformer 204 further inversely discrete-cosine transforms theinversely quantized residual signal. As a result, the residual signal isreconstructed. Next, the information required to generate a predictionsignal is sent to the prediction signal generator 208 through a lineL206. The prediction signal generator 208 generates a prediction signalof the target block based on the information required to generate aprediction signal. A process of generating a prediction signal in theprediction signal generator 208 is described later using FIG. 14. Thegenerated prediction signal is sent to the adder 205 through a line L208and is added to the reconstructed residual signal. The target blocksignal is thus reconstructed and output to the loop filter 209 through aline L205 and, at the same time, stored into the block memory 215 to beused for intra-picture prediction of subsequent blocks. The loop filter209 removes a block noise from the reconstructed signal input throughthe line L205. The reconstructed image having a block noise removed isstored into the frame memory 207 as a reconstructed image to be used fordecoding and reproducing subsequent images.

The prediction signal processing flow performed in the prediction signalgenerator 208 is described using FIG. 14. First, in step S402, theprediction method decoded by the data analyzer 202 is derived.

If the decoded prediction method is inter-picture prediction (stepS403), the motion information (the motion vector and the referencepicture information) decoded by the data analyzer 202 is derived (stepS404). The frame memory 207 is accessed based on the motion informationto derive a reference signal from a plurality of reference images, and aprediction signal is generated (step S405).

If the decoded prediction method is intra-picture prediction (stepS403), the intra prediction mode decoded by the data analyzer 202 isderived (step S406). The block memory 215 is accessed to derivepreviously reconstructed pixel signals located adjacent to the targetblock as reference samples, and a prediction signal is generated basedon the intra prediction mode (step S407). The generated predictionsignal is output to the adder 205 through L208.

The decoding method used in the data analyzer 202 may be arithmeticdecoding or may be variable length decoding.

Next, the intra-picture prediction method in the embodiment of thepresent invention is described using FIG. 3 and FIG. 7. Specifically,the details of step S309 in FIG. 13 and step S407 in FIG. 14 aredescribed, which include a method of estimating the intra predictionsamples in a target block by extrapolation based on the intra predictionmode using the reference samples derived from the block memory 113 inFIG. 1 or the block memory 215 in FIG. 2.

In the present invention, in order to suppress noise such as countouringartifacts described previously in the Technical Problem section, abilinear interpolation process is applied to a group of referencesamples used in the intra-picture prediction with respect to the blockthat suffers contouring artifacts. An appearance of step-like noise atthe block boundary of the reference sample group is suppressed by makingthe signal of the reference sample group smoothly change.

The bilinear interpolation process applied to the reference sample groupis described using FIG. 7. When a target block 210 has a block size ofN×N samples, the neighbouring reference sample group 270 of 4N+1reference samples (ref[x] (x=0 to 4N)) is formed with the previouslyreconstructed signals belonging to five previously reconstructed blocks220, 230, 240, 250, and 260. In the present embodiment, three referencesamples located at the ends of the reference sample 270, namely, thebottom-left reference sample BL=ref[0] and the above-right referencesample AR=ref[4N], and the above-left reference sample AL=ref[2N]located at the center of the reference sample group 270 and to the aboveleft of the target block are defined as key reference samples ofbilinear interpolation. Here, the 4N+1 reference samples areinterpolated as follows.ref′[0]=ref[0]  (1)ref′[i]=BL+(i*(AL−BL)+N)/2N(i=1 to 2N−1)  (2)ref′[2N]=ref[2N]  (3)ref′[2N+i]=AL+(i*(AR−AL)+N)/2N(i=1 to 2N−1)  (4)ref′[4N]=ref[4N]  (5)where, ref′[x] (x=0 to 4N) represents the values of the interpolatedreference samples. Equations (2) and (4) may be transformed to Equation(2)′ and (4)′, respectively.ref′[i]=((2N−i)*BL+i*AL+N)/2N(i=1 to 2N−1)  (2)′ref′[2N+i]=((2N−i)*AL+i*AR+N)/2N(i=1 to 2N−1)  (4)′

The reference sample values between BL and AL are generated with keyreference samples BL and AL by bilinear interpolation, and the referencesample values between AL and AR are generated with key reference samplesAL and AR by bilinear interpolation, resulting in that the levels of theinterpolated reference samples values are made smoothly changed. As aresult, propagation of block noise to the prediction signal can besuppressed.

Next, the criteria for determining whether the bilinear interpolationshould be applied to the reference samples are described using FIG. 7.In the present embodiment, the determination is made using the three keyreference samples and two reference samples at the block boundary, andtwo thresholds. THRESHOLD_ABOVE and THRESHOLD_LEFT are thresholds usedin determining whether the bilinear interpolation should be applied tothe reference samples ref[x] (x=2N+1 to 4N−1) on the upper position andthe reference samples ref[x] (x=1 to 2N−1) on the left position,respectively, with respect to the target block. The bilinearinterpolation is applied to the reference sample that satisfies thedetermination criteria.

In the present embodiment, the determination criteria below are used.Interpolate_Above and Interpolate_Left in the two equations below areBoolean values. When the right side is satisfied, true (1) holds, andthe bilinear interpolation is applied. When the right side is notsatisfied, false (0) holds, and intra smoothing by the conventional 121filter is applied.Interpolate_Left=abs(BL+AL−2*ref[N])<THRESHOLD_LEFT  (6)Interpolate_Above=abs(AL+AR−2*ref[3N])<THRESHOLD_ABOVE   (7)

When the values of BL, AL, and ref[3N] are on a straight line, the valueof BL+AL−2*ref[N] is zero. Similarly, when the values of AL, AR, andref[3N] are on a straight line, the value of AL+AR−2*ref[3N] is alsozero. In other words, the two equations above compare the magnitude ofdeviation of ref[N] from the straight line connecting BL and AL and themagnitude of deviation of ref[3N] from the straight line connecting ALand AR, with the respective thresholds. If the calculated two deviationsare smaller than the corresponding threshold THRESHOLD_ABOVE orTHRESHOLD_LEFT, the Boolean value (Interpolate_Above orInterpolate_Left) is true, and the bilinear interpolation is applied tothe reference sample. In Equations (6) and (7), abs(x) calculates theabsolute value of x.

The values of the two thresholds (THRESHOLD_ABOVE and THRESHOLD_LEFT)may be preset to fixed values, or may be encoded for each frame or foreach slice having a plurality of blocks together, and decoded by thedecoder. The values of the two thresholds may be encoded for each blockand decoded by the decoder. In FIG. 2, the two thresholds are decoded bythe data analyzer 202 and output to the prediction signal generator 208for use in generating an intra-picture prediction signal detailed belowin FIG. 3 and FIG. 4.

FIG. 3 shows a flowchart of a process of estimating the intra predictionsamples by extrapolation (in the directions of intra-pictureprediction). First, in step S510, the prediction signal generator (103or 208, the reference numeral is hereinafter omitted) derives thereference samples ref[x] (x=0 to 4N) as shown in the pixel group 270 inFIG. 7 from the block memory (113 or 215, the reference numeral ishereinafter omitted). If the neighbouring blocks have not yet beenreconstructed because of the encoding order or other reasons, and all ofthe 4N+1 samples cannot be derived, the missing samples are substitutedby the padding process (the values of the neighbouring samples arecopied), whereby 4N+1 reference samples are prepared. The details of thepadding process are described in Non Patent Literature 1. Next, in step560, two Boolean values Interpolate_Above and Interpolate_Left arecalculated with Equations (6) and (7).

Next, in step 520, the prediction signal generator determines whetherthe target block satisfies the determination criteria for applying thebilinear interpolation. Specifically, it is determined whether the sizeof the target block is greater than a predetermined M, and it is alsodetermined whether the calculated Interpolate_Above and Interpolate_Leftare both true. The reason why the block size is set as a determinationcriterion is because the problem of contouring artifacts is likely tooccur in a block of a large size. The test for determining whether thesize of a block is larger than the large value M helps avoid performingunnecessary changes to the reference samples.

If the two determination criteria are satisfied (block size>=M andInterpolate_Above==true and Interpolate_Left==true), the processproceeds to step 530. If not satisfied, the process proceeds to step540. In step 530, the bilinear interpolation process shown by Equations(1) to (5) is applied to the reference samples ref[x] (x=0 to 4N) togenerate the interpolated reference samples ref′[x] (x=0 to 4N). In step540, according to Equations (8) and (9), intra smoothing by the 121filter is applied to the reference samples ref[x] (x=0 to 4N).ref′[i]=ref[i] (i=0 and 4N)  (8)ref′[i]=(ref[i−1]+2*ref[i]+ref[i+1]+2)/4(i=1 to 4N−1)  (9)where ref′[x] (x=0 to 4N) represents the values of the smoothedreference samples.

Finally, in step 550, the intra prediction samples of the target blockare estimated by extrapolation (in the direction of intra-pictureprediction) using the already determined intra prediction mode and theinterpolated or smoothed reference samples ref′[x] (x=0 to 4N).

FIG. 4 further illustrates the details of FIG. 3 and shows a flowchartof a process of estimating the intra prediction sample by extrapolation(in the direction of intra-picture prediction) in a case where theswitching between the bilinear interpolation and the 121 filter iscarried out separately and independently for the left reference samples(ref[x], x=0 to 2N) and the upper reference samples (ref[x], x=2N to4N). First, in step 610, the prediction signal generator (103 or 208,the reference numeral is hereinafter omitted) derives reference samplesref[x] (x=0 to 4N) as shown in the pixel group 270 in FIG. 7 from theblock memory (113 or 215, the reference numeral is hereinafter omitted).If the neighbouring blocks have not yet been reconstructed because ofthe encoding order or other reasons, and all the 4N+1 reference samplescannot be derived, the missing samples are substituted by the paddingprocess (the values of the neighbouring samples are copied), whereby4N+1 reference samples are prepared. The details of the padding processare described in Non Patent Literature 1.

Next, in step 680, the two Boolean values Interpolate_Above andInterpolate_Left are calculated with Equations (6) and (7).

Next, in step 620, the prediction signal generator determines whetherthe target block satisfies the criteria for applying the bilinearinterpolation. Specifically, it is determined whether the size of thetarget block is greater than the predetermined value M, and it is alsodetermined whether at least one of the calculated Interpolate_Above andInterpolate_Left is true. If these two determination criteria aresatisfied (block size>=M and Interpolate_Above true orInterpolate_Left==true), the process proceeds to step 625. If notsatisfied, the process proceeds to step 660. In step 660, intrasmoothing by the 121 filter is applied to the reference sample groupwith Equations (8) and (9).

In step 625, it is determined whether the determination criterion, asshown in Equation (6), for applying the bilinear interpolation for theleft reference samples is satisfied. Specifically, if Interpolate_Leftis true (1), the process proceeds to step 630, and the bilinearinterpolation process shown in Equations (1) and (2) is applied to thereference samples ref[x] (x=0 to 2N) to generate the interpolatedreference samples ref′[x] (x=0 to 2N). If the determination criterion inEquation (6) is not satisfied, the process proceeds to step 635, andintra smoothing by the 121 filter is applied to the left referencesamples ref[x] (x=0 to 2N) with Equations (10) and (11).ref′[0]=ref[0]  (10)ref′[i]=(ref[i−1]+2*ref[i]+ref[i+1]+2)/4(i=1 to 2N−1)  (11)where ref′[x] (x=0 to 2N) represents the values of the smoothedreference samples.

Next, in step 640, it is determined whether the determination criterion,as shown in Equation (7), for applying the bilinear interpolation forthe upper reference samples is satisfied. Specifically, ifInterpolate_Above is true (1), the process proceeds to step 650, and thebilinear interpolation process is applied to the upper reference samplesref[i] (i=2N+1 to 4N) with Equations (3), (4), and (5). If thedetermination criterion in Equation (7) is not satisfied, the processproceeds to step 655, and intra smoothing by the 121 filter is appliedto the upper reference samples ref[x] (x=2N+1 to 4N) based on Equations(12), (13), and (14).ref′[2N]=ref[2N]  (12)ref′[i]=(ref[i−1]+2*ref[i]+ref[i+1]+2)/4(i=2N+1 to 4N−1)   (13)ref′[4N]=ref[4N]  (14)where ref′[x] (x=2N+1 to 4N) represents the values of the smoothedreference values.

Finally, in step 670, the intra prediction samples of the target blockare estimated by extrapolation (in the direction of intra-pictureprediction) using the already determined intra prediction mode and theinterpolated or smoothed reference samples ref′[x] (x=0 to 4N). Forextrapolation, when a line is projected in the direction of intraprediction toward the interpolated or smoothed reference samples fromthe position of the sample in the target block to be extrapolated, theinterpolated or smoothed reference samples that are located close to theprojected line are used.

A moving picture prediction encoding program for causing a computer tofunction as the moving picture prediction encoding device 100 describedabove can be provided in a recording medium. Similarly, a moving pictureprediction decoding program for causing a computer to function as themoving picture prediction decoding device 200 described above can beprovided in a recording medium. Examples of the recording medium includea recording medium such as a USB memory, a flexible disk, a CD-ROM, aDVD, or a ROM, and a semiconductor memory.

For example, as shown in FIG. 16, a moving picture prediction encodingprogram P100 includes a block division module P101, a prediction signalgeneration module P102, a residual signal generation module P103, aresidual signal compression module P104, a residual signal restorationmodule P105, an encoding module P106, and a block storage module P107.

For example, as shown in FIG. 17, a moving picture prediction decodingprogram P200 includes a decoding module P201, a prediction signalgeneration module P202, a residual signal restoration module P203, and ablock storage module P204.

The moving picture prediction encoding program P100 or the movingpicture prediction decoding program P200 configured in this manner isstored in a recording medium 10 shown in FIG. 5 and FIG. 6 describedlater and is executed by a computer described later.

FIG. 5 is a diagram showing a hardware configuration of a computer 30for executing a program stored in a recording medium, and FIG. 6 is anoverview of the computer 30 for executing a program stored in arecording medium. The computer 30 referred to here broadly includes aDVD player, a set-top box, a mobile phone, and the like, which areequipped with a CPU for performing information processing or control bysoftware.

As shown in FIG. 6, the computer 30 includes a reader 12 such as aflexible disk drive, a CD-ROM drive, or a DVD drive, a working memory(RAM) 14 having a resident Operating System, a memory 16 for storing aprogram stored in the recording medium 10, a display device 18 such as adisplay, a mouse 20 and a keyboard 22 serving as input devices, acommunication device 24 for transmitting/receiving data, and a CPU 26for controlling execution of a program. When the recording medium 10 isinserted to the reader 12, the computer 30 can access the moving pictureprediction encoding program stored in the recording medium 10 from thereader 12 and can operate as the moving picture prediction encodingdevice 100 described above with the moving picture prediction encodingprogram. Similarly, when the recording medium 10 is inserted to thereader 12, the computer 30 can access the moving picture predictiondecoding program stored in the recording medium 10 from the reader 12and can operate as the moving picture prediction decoding device 200described above with the moving picture prediction decoding program.

The present invention may take the following modifications:

(A) Determination Criteria for Applying Bilinear Interpolation

The determination criteria for applying the bilinear interpolation arenot limited to the method discussed in the foregoing embodiment. Forexample, supposing that the result of determination for applyinginterpolation is always true, steps 520, 620, 625, and 640 may beomitted. In this case, the interpolation process is always applied inplace of the smoothing process by the 121 filter.

The intra prediction mode may be added to the determination criteria.For example, contouring artifacts at the block boundary are alleviatedby a block noise removing process, and therefore, the result ofdetermination for applying the interpolation process may be always falsewhen the prediction direction of the extrapolation process is verticalor horizontal.

The block size test may be eliminated from the determination criteria.The correlation of block size between the target block and theneighbouring block may be used as a determination criterion in place ofthe block size of the target block. In the example in FIG. 7, the blocksize of the block 260 located adjacent on the left of the target block210 is larger than the target block 210. In this case, a block noisedoes not occur around ref[N]. When the block size of the neighbouringblock is larger than the target block in this manner, the determinationcriterion for applying interpolation may be false irrespective of theresult in Equation (6) or (7). On the other hand, the blocks 230, 240,and 250 located adjacent above the target block 210 are smaller than thetarget block 210. In this case, interpolation application is determineddepending on the result of Equation (6) or (7) because it is possiblethat a block noise occurs around ref[3N] or ref[2N+N/2]. The correlationin block size between the target block and the neighbouring block may beused as a determination criterion together with the block size of thetarget block.

The thresholds (THRESHOLD_ABOVE and THRESHOLD_LEFT) in Equations (6) and(7) may be defined separately for different block sizes and block shapes(differences in block vertical and horizontal sizes) or different intraprediction modes and encoded, and reconstructed by the decoder.Alternatively, the values of THRESHOLD_ABOVE and THRESHOLD_LEFT may beset to the same value, only one of which is encoded and decoded by thedecoder. In the decoder, the threshold reconstructed by the dataanalyzer 202 in FIG. 2 is input to the prediction signal generator 208.In the prediction signal generator 208, the values of Interpolate_Aboveand Interpolate_Left are calculated based on the input threshold (step560 in FIG. 3 or step 680 in FIG. 4).

Instead of providing the determination criteria in steps 520, 620, 625,and 640, the determination result may be included in the bit stream tobe encoded and decoded by the decoder. In this case, in the predictionsignal generator 103 in FIG. 1, the values (0 or 1) of Interpolate_Aboveand Interpolate_Left, the two values, are obtained based on the size ofthe target block and the results in Equation (6) and (7) and are encodedas prediction information necessary to predict each block or each blockgroup consisting of a plurality of blocks. In other words, those valuesare sent to the entropy encoder 111 through the line L112 for encodingand then output from the output terminal 112. When the values (0 or 1)of Interpolate_Above and Interpolate_Left are derived, the correlationof block size between the target block and the neighbouring block andthe size of the target block, and the intra prediction mode as describedabove may be used.

In the data analyzer 202 in FIG. 2, the values of Interpolate_Above andInterpolate_Left are decoded for each block or for each block groupconsisting of a plurality of blocks and are input to the predictionsignal generator 208. Those two values may be separately encoded anddecoded, or the two values may be encoded and decoded as a set.

The process of the intra-picture prediction method performed in theprediction signal generator 208 in FIG. 2 is described using FIG. 15. Inthis case, FIG. 15 replaces FIG. 4. In FIG. 14, in step S406, the valuesof Interpolate_Above and Interpolate_Left decoded together with theintra prediction mode are derived. First, in step 710, the predictionsignal generator (103 or 208, the reference numeral is hereinafteromitted) derives the reference samples ref[x] (x=0 to 4N), as shown inthe pixel group 270 in FIG. 7, from the block memory (113 or 215, thereference numeral is hereinafter omitted). If the neighbouring blockshave not yet been reconstructed because of the encoding order or otherreasons, and all the 4N+1 reference samples cannot be derived, themissing samples are substituted through the padding process (the valuesof the neighbouring samples are copied), whereby 4N+1 reference samplesare prepared. The details of the padding process are described in NonPatent Literature 1.

Next, in step 790, the values of Interpolate_Above and Interpolate_Leftare derived. In step 720, the prediction signal generator determineswhether either Interpolate_Above or the value of Interpolate_Left takesa value “1”. If either takes a value “1”, the process proceeds to step725. If not satisfied, the process proceeds to step 760. In step 760,intra smoothing by the 121 filter is applied to the reference samplegroup with Equations (8) and (9).

In step 725, if the value of Interpolate_Left is “1”, the processproceeds to step 730, and the bilinear interpolation process shown inEquations (1) and (2) is applied to the reference samples ref[x] (x=0 to2N) to generate the interpolated reference samples ref′[x] (x=0 to 2N).If the value of Interpolate_Left is “0”, the process proceeds to step735, and intra smoothing by the 121 filter is applied to the leftreference samples ref[x] (x=0 to 2N) with Equations (10) and (11).

Next, in step 740, if the value of Interpolate_Above is “1”, the processproceeds to step 750, and the bilinear interpolation process is appliedto the upper reference samples ref[i] (i=2N+1 to 4N) with Equations (3),(4), and (5). If the value of Interpolate_Above is “0”, the processproceeds to step 755, and intra smoothing by the 121 filter is appliedto the left reference samples ref[x] (x=2N+1 to 4N) with Equations (12),(13), and (14).

Finally, in step 770, the intra prediction samples of the target blockare estimated by extrapolation (in the direction of intra-pictureprediction) using the decoded intra prediction mode and the interpolatedor smoothed reference samples ref′[x] (x=0 to 4N).

(B) Interpolation Process

In the description above, the bilinear interpolation is used in theinterpolation process. However, another interpolation process may beused as long as a noise at the block boundary can be removed. Forexample, all the reference samples may be replaced with the mean valueof the key reference samples. The interpolation process method may bechanged according to the block size or the intra-picture prediction typeThe interpolation process method to be applied may be included in thebit stream to be encoded and decoded.

(C) Process Flow of Intra-Picture Prediction of Reference Samples

The flow of the process of estimating the intra prediction samples byextrapolation (in the direction of intra-picture prediction) is notlimited to the procedure in FIG. 4. For example, steps 625, 630, and 635and steps 640, 650, and 655 may be switched in their order. Equation (3)and Equation (12) may be carried out not in steps 650 and 655 but insteps 630 and 635. Since the process results of Equations (1), (3), and(5) and Equations (10), (12), and (14) are the same, they may be carriedout together immediately before step 625 (between steps 620 and 625) orimmediately after steps 650 and 655 (between step 650 or 655 and step670).

The determination criteria in step 620 may only include the block size.In this case, Equation (12) may be replaced with Equations (15) and (16)because the process result is the same as that of FIG. 4.ref′[2N]=ref[2N]if Interpolate_Above==true∥Interpolate_Left==true  (15)ref′[2N]=(ref[2N−1]+2*ref[2N]+ref[2N+1]+2)/4 others  (16)where ref[2N] represents the values of the smoothed reference sample.(D) Block Size

In the description above, the target block is a square block. Theinterpolation process for the reference samples according to the presentinvention can be applied to a non-square block. An example of a targetblock 290 of a block size of N×2N is shown in FIG. 12. In this case, thenumber of ref[x] is 3N+1.

(E) Key Reference Sample

In the description above, the three key reference samples are located atthe ends and the center of the reference sample group. However, thenumber and the position are not limited thereto. For example, the numberor position may be changed according to the size of the reference blockor the correlation between the reference block and the neighbouringblock. The number and position of the key reference samples may also beincluded in the bitstream to be encoded and decoded. The three keyreference samples at the ends and the center of the reference samplegroup may be set as defaults, and whether to use the defaults or otherkey reference samples may be encoded as instruction information anddecoded. In the data analyzer 202 in FIG. 2, the key reference samplesare updated. As the key reference samples to be updated, ref[N+N/2] andref[2N+N/2] may be added in FIG. 7 or may be used in place of ref[2N].Alternatively, ref[N/2] and ref[3N+N/2] may be used in place of ref[0]and ref[4N], and the 121 filter may be applied to ref[1] to ref[N/2−1]and ref[3N+N/2] to ref[4N−1].

(F) Equations of Determination Criteria

The determination equations used in steps 520, 620, 625, and 640 are notlimited to Equations (6) and (7). For example, ref[N+1] and ref[3N+1]may be used in place of ref[N] and ref[3N] in FIG. 7.

What is claimed is:
 1. A moving picture prediction decoding devicecomprising: a processor configured to: decode, from encoded compressiondata for a plurality of divided blocks, a compressed residual signal,and an intra prediction mode indicating an intra-picture predictionmethod of a target block to be decoded; generate an intra-pictureprediction signal of the target block using the intra prediction modeand previously reconstructed reference samples located adjacent to thetarget block; restore a reconstructed residual signal of the targetblock from the compressed residual signal; and restore a pixel signal ofthe target block by addition of the intra-picture prediction signal ofthe target block to the reconstructed residual signal of the targetblock, and store in a memory the restored pixel signal of the targetblock for use as reference samples, wherein the processor is furtherconfigured to derive a group of reference samples from a plurality ofpreviously reconstructed blocks stored in the memory that neighbor thetarget block, perform interpolation between a first key reference samplelocated at an end of the group of reference samples and a second keyreference sample located at a center of the group of reference samplesand perform interpolation between the second key reference sample and athird key reference sample located at another end of the group ofreference samples to generate interpolated reference samples within thegroup of reference samples, wherein the first, second and thirdreference samples include samples located adjacent to an upper and leftside of the target block, and the processor is further configured toextrapolate the interpolated reference samples based on the intraprediction mode to generate an intra-prediction signal, and theprocessor further configured to selectively carry out interpolationbetween the first key reference sample and the second key referencesample and between the second key reference sample and the third keyreference sample or a smoothing process of the reference samples, basedon a comparison between a predetermined threshold and a value determinedbased on at least one of the first, second or third key referencesamples.
 2. A moving picture prediction decoding method executed by amoving picture prediction decoding device, the moving picture predictiondecoding method comprising: a decoding step of decoding, from encodedcompression data for a plurality of divided blocks, a compressedresidual signal and an intra prediction mode indicating an intra-pictureprediction method of a target block to be decoded; a prediction signalgeneration step of generating an intra-picture prediction signal usingthe intra prediction mode and previously reconstructed reference sampleslocated adjacent to the target block; a residual signal restoration stepof restoring a reconstructed residual signal of the target block fromthe compressed residual signal; and a block storage step of restoring apixel signal of the target block by adding the intra-picture predictionsignal of the target block to the reconstructed residual signal of thetarget block, and storing the restored pixel signal of the target blockfor use as reference samples, wherein the prediction signal generationstep further comprises: deriving a group of reference samples from aplurality of previously reconstructed blocks that are stored andneighbor the target block, interpolating between a first key referencesample located at an end of the group of reference samples and a secondkey reference sample located at a center of the group of referencesamples, and interpolating between the second key reference sample and athird key reference sample located at another end of the group ofreference samples for generating interpolated reference samples withinthe group of reference samples, wherein the first, second and thirdreference samples include samples located adjacent to an upper and leftside of the target block, and the prediction signal generation stepfurther comprising extrapolating the interpolated reference samplesbased on the intra prediction mode to generate the intra-predictionsignal, the prediction signal generation step further comprisesselectively carrying out interpolation between the first key referencesample located at the end of the group of reference samples and thesecond key reference sample located at the center of the group ofreference samples and between the second key reference sample and thethird key reference sample located at the another end of the group ofreference samples, or a smoothing process of the reference samples basedon a comparison between a predetermined threshold and a value determinedbased on at least one of the first, second or third key referencesamples.